Turning off building lights at night can save migrating birds from crashing into buildings, a study based on decades of research has shown.
Scientists found that on nights when half the windows of a large building in Chicago were darkened, there were 11 times fewer bird collisions during spring migration and six times fewer collisions during autumn migration than when all the windows were lit.
In 1978 David Willard, the collections manager emeritus of the Field Museum in Chicago, heard an offhand remark about birds hitting McCormick Place, North America’s largest convention centre, about a mile south of the museum.
“At the time I was only recently hired by the Field Museum. I remember having a casual conversation with somebody who mentioned birds sometimes fly into the windows of this building. I was curious. I thought to myself: I work in the museum and things are dying out there, so why not make use of out of them?” Willard said.
“I walked down to the building early one morning and I found three or four dead birds, some of them already insect-ridden. That piqued my interest. We started going more frequently, and then eventually every single day, so that we knew exactly what nights the birds flew into the buildings.
“I might not have gone back if I hadn’t found anything that first day, and now here we are, 40 years later and 40,000 birds later.”
Willard said some days he and his colleagues would find no dead birds, while occasionally they would find up to 200. “That has happened maybe four times over the whole duration of the 40 years. Zero is much common than 200.”
Willard collected the dead birds and brought them back to the museum, where he recorded each one in a ledger and added them to the museum’s collection.
It was only 20 years later that Willard began to notice a pattern. On nights when the lights were out at McCormick Place, there were fewer birds on the ground the next morning. As the building’s lighting patterns began to vary more, he gathered data on which windows were illuminated each night, in addition to collecting the birds.
Benjamin Van Doren, a postdoctoral associate at the Cornell Lab of Ornithology and the paper’s lead author, helped combine Willard’s specimens and lighting observations with other data on conditions that might play a role in bird mortality, including weather records and radar data.
He said: “Our research provides the best evidence yet that migrating birds are attracted to building lights, often causing them to collide with windows and die.”
He and other scientists built a statistical model that estimated that halving the amount of lighted windows could decrease collision counts up to 11 times during spring and six times in the autumn season. Turning out half the lights during migration seasons could reduce bird mortality at McCormick Place by 59%
He said: “As an ecologist I’m used to studies that are not as clearcut as experiments. In experiments you can control the variables, but in the real world things can be really complex.
“I was really blown away by the sheer magnitude of the influence that building lights may have on birds that we detected. It speaks to the exciting potential to save birds simply by reducing light pollution.”
Birds use unique exploratory skills. Building lights, especially on overcast nights, can bewilder them, leading them to fly in circles. Some may crash into buildings if they recognise plants or trees reflected in the glass.
The US has lost more than a quarter of its bird numbers, or about 3 billion birds, over the past half a century.
“Buildings all across North America, all across the world, are killing birds, and those add up,” said Doug Stotz, a senior conservation ecologist at the Field Museum. I hope this paper will show why it’s important to turn off internal lighting as well, especially in Chicago, which is the country’s deadliest city for migrating birds.”
Van Doren hopes the research can bolster efforts already made by the Lights Out programme in Chicago, which encourages owners and managers of tall buildings to turn off lights to stop migratory disruption, and asks residents of Chicago to do the same.
He said: “Our study contains a hopeful message: we can save birds simply by turning off lights during a handful of high-risk days each spring and fall.”